J+A CrossflowTM Grit Removal

The J+A Crossflow™ grit removal unit is typically installed between medium and fine screens to capture abrasive material. It features a wide and shallow square collection tank with a series of vertical baffles to assist in the equal distribution of the flow entering the tank. This design forms a recessed circular sump. Solids accumulate herein and are transported by a scraper to a collection hopper.

The grit collecting mechanism is powered by a drive unit (with standard worm gear) supported on steel beams spanning the settlement chamber. All drive units incorporate a torque actuator, making it virtually impossible to stall the machine. High grit loadings can be handled with comfort while distributing the grit over a period of time. We can accommodate high grit loading with optional double-wide rakes.

Operation is most efficient when velocity is maintained at a specific value over the whole range of flows. At this velocity, grits of the specified size and specific gravity will be collected. Sizing, positioning and hydraulic control are of the utmost importance.

The sewage flow enters the concrete grit settlement chamber via a series of inlet baffles where hydro-separation takes place and grit settles to the bottom. The independently adjustable inlet baffles provide an even flow distribution and uniform velocity across the entirewidth of the unit.

The sewage enters the mechanism flowing in straight lines across the tank and then over a weir, with free discharge extending along the entire outlet side of the tank. The sludge collection apparatus consists of two or three structural steel arms attached to a vertical shaft and fitted with outward raking blades with scoops on each end. The collected grit is transported to a submerged grit discharge chute and into the grit cleaning unit or grit collection chamber.

The reciprocating rakes move the grit up the tank inclineand with a rolling action that releases the entrained organics. The tank incline extends above the water level where the excess moisture drains off. Grit is discharged into a collection skip, suitable for disposal.

The organics pump transfers any floating material to the settlement chamber via a 200mm NB duct and is transported to the outlet.

Alternatively, if site conditions are constrained, the grit classification section can be altered to a grit pump and screw classifier system. This design gives more flexibility in its arrangement i.e. the height and depth of the installation, and the point of dewatered grit collection.

Which is the best grit removal system – the Crossflow™ Detritor® or Jeta® grit trap?

Both types of grit removal system provide good grit removal efficiencies so there is no clear leader between the two systems. The preferred choice is generally made by the principal engineer.

Do we need to supply a washwater or air supply?

We do not require the supply of washwater or air. The Crossflow Detritor and rake classifier systems do not require washwater to remove organic material. Organic material breaks away from the grit particles with the motion of the cake classifier. An air supply is generally required for either an airlift pump system or for grit agitation. Our system does not require such items.

Can the Crossflow Detritor utilize a free standing grit classifier?

The reciprocating rake classifier is commonly supplied with this type of grit removal system, however we can offer free standing screw or rake classifiers. The advantage of this option being the location of the classifier. In order to transport the collected grit to the classifier we can offer duty or duty / standby grit pumps commonly rated for 11 liters per second. The final pump capacity being subject to site conditions.

What is the water depth at maximum flow rate?

The water depth in the grit collector section will be dictated by the downstream flume. For guidance purposes only the water depth would be approximately one tenth of the chamber diameter.